1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to an organic electroluminescent device having a red luminescent layer of which luminescent efficiency is greatly improved.
2. Discussion of the Related Art
Lately, demands for flat panel displays occupying less space are increased in accordance with the trend of increasing a screen size of a display device. One of the flat panel displays is called an organic light emitting diode (OLED) or an organic electroluminescent display. And, technology for the organic electroluminescent display is developed rapidly, whereby various prototypes have been in market already.
The organic electroluminescent device emits light in a manner that electric charges are injected in an organic layer formed between a hole injection electrode (anode) and an electron injection electrode (cathode) so as to form a pair of electron and hole to generate an exciton and an excited state of the exciton falls to a ground state so as to emit light.
The organic electroluminescent device has excellent characteristics of wide viewing angle, fast response, high contrast, and the like, thereby being applicable to a pixel of a graphic display, a television video display, a pixel of a surface light source, and the like. Moreover, the organic electroluminescent device can be formed on a flexible transparent substrate such as a plastic substrate and made to be thin and light as well as shows an excellent color tone. Hence, such an organic electroluminescent device is suitable for the next generation flat panel display (FPD).
Moreover, the organic electroluminescent device enables to express three colors including green, blue, and red, needs no backlight of a well-known liquid crystal display (LCD) so as to show less power consumption, and has the excellent color tone. Therefore, many concerns are focused on the organic electroluminescent device as the next generation full color display.
A process of fabricating a conventional organic electroluminescent device is explained in brief as follows.
(1) First of all, an anode is formed on a transparent substrate. In this case, ITO (indium tin oxide) is used as an anode material.
(2) A hole infection layer (HIL) is formed on the anode. CuPC (copper phthaiocyanine) is mainly used as the hole injection layer formed 10˜30 nm thick.
(3) A hole transport layer (HTL) is formed. The hole transport layer is formed of NPB{4,4′-bis[N-(1-naphthyl) -N-phenylamino] -biphenyl} so as to be 30˜60 nm thick.
(4) An organic light-emitting layer is formed on the hole transport layer. In this case, a dopant is added thereto if necessary. For instance, in case of green luminescence, Alq3{tris(8-hydroxy-quinolate)aluminum} is used as the organic light-emitting layer so as to be formed 30˜60 nm thick. And, MQD (N-methylquinacridone) is used as the dopant.
(5) An electron transport layer (ETL) and an electron infection layer (EIL) are formed successively on the organic light-emitting layer, or an electron injection transport layer is formed thereon.
In green luminescence, Alq3 used in the organic light-emitting layer has an excellent capability of electron transport, whereby the electron injection/transport layer(s) may not be formed additionally.
(6) Finally, a cathode is formed, and then a passivation layer is formed.
In such a structure, each of blue, green, and red luminescent devices can be realized in accordance with how the organic light-emitting layer is formed. Yet, red luminescence of high efficiency has difficulty in being realized.
Generally, CuPC having excellent hole injection capability and thermal stability is used as a hole injection layer in OELD (organic electroluminescent display).
The thicker CuPC is formed, the more hole injection occurs. Yet, a color is shifted strongly to blue.
Hence, in luminescence of a full color OELD, red wavelength is absorbed so as to reduce an efficiency of red greatly.
If CuPC fails to be used as the hole injection layer, low Tg (glass transition temperature) of the hole transport layer brings about degradation between the anode and hole transport layer so as to decrease durability of the device.
Moreover, when CuPC is formed 10˜60 Å thick, absorption of red wavelength hardly occurs but hole injection is degraded so as to reduce an efficiency of the device greatly.